1. Technical Field
The present invention generally relates to alkali metal thermal to electric conversion (AMTEC) cells and more particularly to means for controlling the alkali metal evaporation front position in an evaporator of the cell.
2. Discussion
An AMTEC cell is a thermally regenerative concentration cell typically utilizing sodium or potassium as a working fluid and a beta-alumina type solid electrolyte as an ion selective membrane. The electrolyte permits a nearly isothermal expansion of sodium to generate high-current/low voltage power at high efficiency. Most AMTEC cells employ at least one beta-alumina type solid electrolyte (BASE) element which is exposed to high-pressure sodium on an inner surface and low-pressure sodium on an outer surface.
The BASE element's inner and outer surfaces are overlaid with permeable electrodes which are connected to each other through an external load circuit. Neutral sodium atoms incident on the BASE tube's inner surface give up their electrons at the inner electrode (the anode). The resulting sodium ions pass through the tube wall under the applied pressure gradient, and the emerging sodium ions are neutralized at the outer electrode (the cathode) by electrons returning from the external load. Thus, the pressure gradient drives the sodium through the BASE element thereby creating an electrical current which passes through the external load resistance.
The neutral sodium atom vapor leaving the outer electrodes flows through the space between the BASE elements and the cell wall until it condenses at a low-temperature condenser at the cold end of the cell. From there, the sodium condensate flows through an artery containing a fine pore membrane consisting of a densely packed stainless steel wire matrix, similar to those used in heat pipe wicks. The liquid sodium evaporates to a high pressure at the bottom of an evaporator wick which is coupled to the artery membrane. The high-pressure sodium vapor is returned to the insides of the BASE elements through a common plenum at the bottom of the cell.
Some cells employ multiple BASE tubes with sodium vapor on both sides of the tube wall to prevent shorting of the BASE tubes within each cell. The inner surface of each BASE tube is exposed to high-pressure sodium vapor and the outer surface is exposed to low-pressure sodium vapor. The high-temperature evaporator near the hot end of the cell produces the high pressure and the low-temperature condenser at the cold end of the cell maintains the low-pressure.
As the AMTEC cell experiences different operating load conditions and thermal environments, the amount of required current flow changes. In response, the sodium mass flow rates and the temperatures within the cell change significantly. As such, the optimal position within the evaporator for the location of the evaporation front varies. Controlling this sodium evaporation front position within the evaporator over a great range of operating conditions is a critical concern. By controlling the front position, reliable and robust AMTEC cells capable of performing across a broad range of operating load conditions and thermal environments may be produced.
Conventional AMTEC cells only operate at peak performance at a single set of operating conditions which coincide with a single evaporation front position. For instance, the cell may be designed to operate at two amps. At this current, the temperature at the end of the evaporator (e.g., 800.degree. C.) is ideal for sustaining sodium evaporation. However, as the current load increases or decreases, the temperature at the end of the evaporator changes (e.g. .+-.100.degree. C.). As a result, the position of the liquid/vapor interface (i.e., evaporation front) moves to a new position within the evaporator. This part of the evaporator is at the appropriate temperature for evaporation at that new current or operating condition. However, since this location is no longer at the end of the evaporator, the resulting sodium vapor must migrate through the evaporator wick material before entering the free space below the evaporator. This results in poor cell performance. As such, conventional AMTEC cells are not flexible to changing operating conditions typical of many power conversion/generation applications. In order to compensate, some prior art cells employed evaporator wicks which provided excess wicking capability in an effort to prevent movement of the evaporation front. However, this caused poor AMTEC cell performance due to dramatically reduced sodium vapor pressure and mass flow due to low permeability, or increased cell manufacturing costs to produce very small pore sizes.
In view of the drawbacks of conventional cells, it is desirable to provide an AMTEC cell employing an evaporator capable of accommodating an alkali metal evaporation front position moving in a prescribed, well-characterized manner in response to different alkali metal flow rates and changing thermal conditions, or maintaining alkali metal evaporation front position during different alkali metal flow rates and changing thermal conditions.